Headlamps of motor vehicles are made to switch between a high beam for illuminating a far-field area as well as a near-field area ahead of the subject vehicle and a low beam for illuminating ahead of the subject vehicle while preventing the dazzling of other vehicles such as oncoming vehicles and vehicles traveling ahead of the subject vehicle. However, the high beam and the low beam have their own light distribution patterns which are significantly different. The high beam illuminates mainly a road area which lies ahead of and square to the vehicle, whereas the low beam illuminates a near-field area directly in front of the vehicle and excludes the area which lies ahead of and square to the vehicle. Thus, the illuminating conditions ahead of the subject vehicle change significantly when the headlamps are switched from the high beams to the low beams or vice versa, and this causes the driver to feel a sensation of physical disorder or makes safe driving difficult. For example, when the headlamps are switched from the high beams to the low beams, since the illuminating conditions are switched from a state in which the wide area spreading ahead of the subject vehicle is illuminated with high luminance intensity to a state in which only the near-field area direct in front of the subject vehicle is illuminated with a lower luminance intensity than the high luminance intensity, the peripheral areas to the near-field area and the far-field area ahead of the subject vehicle get dark, whereby visibility is lowered, causing the driver to feel uneasy. In contrast, when the headlamps are switched from the low beams to the high beams, other vehicles traveling on the periphery of the near-field area or in the far-field area which have not been recognized by the driver of the subject vehicle until then due to insufficient illumination by the low beam are now illuminated with the high beam. As this occurs, drivers of the other vehicles are dazzled by the high beam of the subject vehicle, making safe driving difficult.
To deal with these problems, Japanese Patent Document JP-A-2002-19517 proposes a technique for enhancing far-field visibility of the subject vehicle while preventing the dazzling of preceding vehicles by configuring headlamps to control directions of optical axes of low beams, measuring an inter-vehicle distance between the subject vehicle and a preceding vehicle and deflecting vertically the optical axes of the low beams in accordance with the inter-vehicle distance so measured. In addition, Japanese Patent Document JP-A-2004-161082 proposes a technique for enhancing the forward visibility of the subject vehicle without dazzling the drivers of other vehicles by detecting other vehicles by a vehicle detection means, and independently controlling quantities of light of high beams and low beams of left and right headlamps based on a relative distance to other such vehicles.
Although the technique proposed by JP-A-2002-19517 can improve the far-field visibility of the subject vehicle by the illumination of the low beams, the technique does not eliminate the sensation of physical disorder that is felt by the driver when the headlamps are switched between the high beams and the low beams as has been described above and does eliminate dazzling of other vehicles. Although the technique proposed by JP-A-2004-161082 can illuminate the high beams and the low beams without dazzling other vehicles, the technique cannot solve the problems with the sensation of physical disorder and dazzling of other vehicles which are caused when the headlamps are switched between the high beams and the low beams.